Molecular ion imaging and dynamic secondary-ion mass spectrometry of organic compounds

Abstract

An ion microscope equipped with a resistive anode encoder imaging system has been used to acquire molecular secondary ion images, with lateral resolution on the order of 1 microns, from several quaternary ammonium salts, an amino acid, and a polynuclear aromatic hydrocarbon which were deposited onto copper transmission electron microscope grids. All images were generated by using the secondary ion signal of the parent molecular species. The variation of parent and fragment molecular ion signals with primary ion dose indicates that, for many bulk organic compounds, bombardment-induced fragmentation of parent molecules saturates at primary ion doses of (1-8) X 10(14) ions/cm2. Subsequent ion impacts cause little further accumulation of damage in the sample, and intact parent molecular ions are sputtered even after prolonged ion bombardment (i.e. primary ion doses greater than 1 X 10(16) ions/cm2). This saturation process allows molecular images to be obtained at high primary ion doses and allows depth profiles to be obtained from simple molecular solid/metal test structures.

Energy Deposition during Molecular Depth Profiling Experiments with Cluster Ion Beams

“…The first term in eq 1 describes the supply of undamaged molecules from the bulk into the altered layer due to the erosion process, while the second and third terms describe the loss of intact molecules from the altered layer due to sputtering and chemical damage, respectively. Ideally, eq 1 results in an exponential decay from an initial signal S 0 at zero fluence toward a steady state signal S ss with a disappearance cross section σ=Y tot /nd + σ D , yielding (2) Since the degree of chemical damage decreases as S ss approaches S 0 , the relationship Y tot >> ndσ D describes favorable conditions for molecular depth profiling. That is, chemical information is most effectively maintained during sample erosion when the total sputtering yield Y tot (the number of cholesterol molecules removed per impact) is large relative to the number of damaged molecules ndσ D within the altered layer.…”

“…For example, trehalose films dosed with small peptide molecules were eroded with Au + , Au 2 + , and Au 3 + clusters at 20 keV as well as with C 60 + with kinetic energies ranging from 20 to 120 keV. 14,18,29,30 While each projectile except for Au + was shown to maintain a molecular ion signal during erosion, the C 60 + projectile at 40 keV produced the cleanest depth profile.…”

Use of High Lateral Resolution Secondary Ion Mass Spectrometry to Characterize Self-Assembled Monolayers on Microfabricated Structures

“…SIMS has already been shown to yield reliable "fingerprint" spectra of molecular materials, 7 -11 and high lateral resolution imaging of both inorganic and organic materials has been demonstrated. 12 , 13 Characterizations of vapor deposited organic films, 10,1I Langmuir-Blodgett assemblies, [14][15][16][17][18] and silated surfaces 19 have also been reported, verifying the ability of SIMS to detect monolayer quantities of organic materials. …”

Development of a Primary Ion Column for Mass Spectrometry-Based Surface Analysis

“…However, the low sputter yields associated with common projectiles coupled with the poor ionization efficiency yields low number of thus damage to the sample is far more pronounced, due to irreversible changes in the morphology of the sample [9][10] . Conventional atomic projectiles has been known to lead to loss of all molecular information, generally relegating the technique to elemental and small molecules 18 . Yet in this operational mode, one can thoroughly profile the chemical make-up of the sample as a function of depth.…”

“…Currently, many of these sources are still utilized in commercial instrumentation (e.g., CAMECA ion microprobes); however their use has been largely eclipsed by polyatomic projectiles for molecular analyses. Thermal Ionizers were reserved to the usage of metal salts with low melting points such as cesium iodide, where the salt housed in a reservoir was heated and directed by an electrostatic potential on a fritted film, where the ions are emitted from 18 .…”